The present invention relates to a combustion grate, having a modular frame, installed in the furnace of a waste disposal plant, in which the aforesaid waste is burnt and so disposed of as ashes.
Such disposal generally is associated to an energy recovery system through the production of overheated steam and the exploitation of steam in a turbine, which in turn is coupled to an electric generator.
Such plants generally comprise a combustion chamber inside which the waste, laid onto a combustion grate, is burnt, through which an adequate quantity of air is inputted. The combustion grate is adapted to sustain and move forward the waste during the combustion permitting at the same time the insufflation of combustion air under the waste bed. The grate forms the lower portion of the combustion chamber. The combustion chamber begins physically immediately over the grate. In some cases, the walls of the combustion chamber are completely or partially cooled, by means of evaporating tube bundles protected by the refractory itself.
The region at the interface between grate and combustion chamber is made by the refractory-carrying beam. Inside the combustion chamber, the flame produced by the combustion of waste is spread, reaching temperatures over 1400° C. The surface of the grate is hit only occasionally by the radiation of the flame, as it is normally protected by the waste bed in transit.
The surface of the grate is made by plates (typically called “fire bars”) which are normally made of molten steel having a high chromium content, in order to show high wear characteristics when hot. The advancement of waste is obtained through the relative movement of the fire bars which can have several characteristics. The actuation system is normally made of hydraulic pistons. The fire bars are provided with apertures or holes to allow the combustion air to flow from underneath the plane of the grate, through the waste. The combustion air has in fact the double function of providing the oxygen for the oxidation of the waste and of cooling the fire bar by maintaining it at an acceptable temperature in order to maintain the mechanical characteristics. The cooling is necessary, as the grates normally work covered by the forwarded fuel, but they can also be directly exposed to the combustion flames.
The steps making the grate can also be provided with an additional cooling with water, particularly when they are used for the combustion of fuels with high calorific power. Such cooling is obtained by a liquid circulation which is forced to lap the surfaces non contacting the fuel of every fire bar, through a liner or an equivalent apparatus for the accumulation of liquid.
The fire bars at the initial and terminal ends of every step are separated from the carrying structure of the grate by means of plates, generally of the same material of the fire bar, which are kept together with pressure against the side of the fire bar itself. Such plates have the function of laterally containing the fuel forwarded in the region immediately above the fire bars, and of separating it from the lateral portions of the grate, which do not tolerate the direct exposition to the burning material. The lateral plates eventually join the plane made by the fire bars with the vertical surfaces of the combustion chamber, which are disposed immediately above the grate.
The grate further comprises a plurality of handling groups, each formed by the cited fire bars organized in bundles, which relatively move one with respect to the other advancing the waste on the grate. In particular, the fire bars are divided in fixed and movable fire bars, which through slides cause a back-and-forth movement, by sliding one on another and determining the advancement of the waste in each handling group, and so in general on the grate.
The disposal of the solid urban waste, even if it is not characterized by a highest technological content, is an activity particularly sensible from a reliability point of view and the guarantees of the function. The complex integrated system of the waste disposal (the accumulation, the collection, the transport, the stockpiling and the disposal) requires that the technological components employed in the last stage of the supply chain permit a continuous operation over 24 hours and minimize the risks of stopping due to damages (minor or catastrophic damages as they can be). The operator of the disposal plant requires reliability robustness and simplicity of the component “grate”, in the operation and in the maintenance.
Patent application MI2004A001746 describes a plant of this kind having a movable combustion grate, in which the handling group is formed by a plurality of fire bars or plates, which alternatively move one with respect to the other by putting forward the waste on said grate, which realizes a substantially horizontal and at least partially continuous combustion plane.
The movable fire bars are bound to a movable frame, which is pushed by two pistons, one on each side. The fixed fire bars are in turn connected with a fixed frame, common to all fixed fire bars.
The relative movement between the fixed frame and the movable frame is of a simple alternate translation.
The grate is further provided with sliding elements adapted to determine the sliding between the two frames and the movement limiting elements between them which determine its stroke.
In the forward stroke the movable fire bar pushes the waste on the back of the fixed fire bar until causing its fall onto the subsequent movable fire bar, and at the same time it drags the waste on its own back. In the return stroke, the waste on the back of the movable fire bar finds an obstacle on the front of the fixed fire bar and, instead of moving back, it is pushed onto the back of the fixed fire bars downwards and then it is pushed forwards in the subsequent stoke, so determining the advancement of the waste in each handling group on the grate in general.
The sliding elements are substantially made by bearings or rolls upon which a pad slides substantially integrally with the moveable frame with an inclination dependent on the direction of movement which the moveable frame must communicate to the fire bars.
The movement limiting elements comprise a track integral with the carrying or fixed frame, whereas to the movable frame of the grate two wheels are connected, bound to a fixed axis. The wheels are mounted with a transversal clearance with respect to the track. When the movable frame moves in different directions with respect to that required for the correct feeding of the fire bar, the clearance between the wheels and the frame is reduced until becoming zero by stopping the movement of the movable frame.
The movable frame is usually handled by hydraulic pistons with interposed crank gears and traditionally it is made by a carrying frame, made of longitudinal and transversal beams. To the upper surface of the longitudinal beams fire bar-carrying beams are fixed, on which the same fire bars rest. To the lower surface of the longitudinal beams the pads are instead fixed, which slide on the rolls of the sliding elements by giving to the mobile frame the correct direction of movement.
Such technological solution has a remarkable constructive complexity. In fact, the longitudinally disposed beams perform just structural functions. Therefore they must be predisposed, by means of reinforcement plates, for the fastening of the fire bar-carrying beams, of the sliding pads and of support structures for bonding the hydraulic pistons.